This invention relates to electrodes and is particularly related to electrodes which are suitable for use in electrolytic processes. Examples of such electrolytic processes are chlor-alkali electrolysis, electroplating and cathodic protection.
This invention is particularly concerned with electrodes in which at least the surface of an electrode base is formed of a "film-forming metal", there being applied to at least part of said surface an electrically conductive electrolyte-resistant and electrolysis product resistant coating. The term "film-forming metal" is used herein to refer to titanium and titanium base alloys, tantalum and tantalum base alloys, zirconium and zirconium base alloys, niobium and niobium base alloys, hafnium and hafnium base alloys. By "metals of the platinum group" is meant platinum, iridium, rhodium, osmium, ruthenium and palladium, and alloys thereof.
There has been proposed, see for example British Patent Specification No 925080, a method of manufacturing an electrode composed of a core of titanium and a porous coating of a metal of the platinum group. The titanium core was provided with a barrier layer by anodising or by oxidation before the coating was applied to it. The British Patent Specification refers to the advantages of such a method, stating them to be the avoidance of any necessity prior to coating with a metal of the platinum group to have to remove the oxide film naturally occurring on titanium. Further advantages are said to be the certainty that the titanium will be protected from corrosion by the barrier layer, even under the coating of a metal of the platinum group, which could be significant should said coating be damaged, the avoidance of any need to remove the barrier layer when a fresh coating of the platinum group is to be applied, and the ease in providing an adherent coating of the metal of the platinum group.
In British Patent Specification No 1327760, there is described an improved method of applying a barrier layer onto the film-forming metal. Basically, the method comprises inserting a film-forming metal surface into a solution of titanium and depositing an oxide of titanium onto the film-forming metal surface. An electrically conductive and electrolyte-resistant layer is then applied to the titanium oxide surface.
It has now been discovered that a great improvement in the method of manufacturing an electrode can be obtained by depositing more than one oxide layer from a solution and heating each oxide layer above ambient temperature to dry out the layer thoroughly before applying any further oxide layer to the surface. This change in procedure leads to a significant increase in the durability of the coating.
Without prejudice to the present invention, it is believed that heating the oxide layer above ambient temperature causes it to crack as the moisture contained in the layer is driven off. Any subsequent layers which are applied and heated also crack, but since the cracking is at random, there is a reasonable possibility that the cracks will not coincide. The effect of this is to reduce the direct path between the outer surface of the eventual electrode and the film-forming metal substrate. Clearly, if more than two layers are used, the probability of a direct path is further reduced. If the oxide layers are not dried above ambient temperature, however, the moisture is retained and the oxide layer does not produce anything more than incipient cracking. This means that any substrate oxide layer applied is effectively continuous with the first layer and when heated above ambient temperatures, both layers crack as a single unit.